Fungicides have been categorized into groups based on
their mode of action against plant pathogenic fungi. These groups are
numbered and contain all fungicides with similar modes of action, whether
or not they are in the same chemical class. Pathogens that develop
resistance to one fungicide in a group are likely to be resistant to
all fungicides in that group. Therefore, most label instructions for
fungicides at risk for resistance development require alternating such
fungicides with fungicides with a different mode of action. The fungicide
group number is provided on the label and is shown in the table below.

When deciding on which fungicides to use in alternation, choose fungicides
that are: 1) effective against the diseases of concern, and 2) have different
Fungicide Group numbers. Some products are a mixture of fungicides from two
different groups, e.g. Groups 11 and 27 (Tanos); Groups 9 and 12 (Switch);
and Groups 7 and 11 (Pristine). Nonetheless, label instructions for these products
require alternation with additional fungicides.

Fungicide group number and risk of resistance development for some fungicides
commonly used in vegetable disease management. Adapted from FRAC Fungicide
List (1) (2003) (http://www.frac.info/publications/frac_list02.html). Not all
fungicides labeled for use on vegetables are included, and not all are labeled
for use on tomatoes. Check the label for permitted uses.

Editor’s Note: Dr. Sally A. Miller is a professor,
Department of Plant Pathology, Ohio State University. She can be
contacted by phone at
(330) 263-3678 or by e-mail at miller.769@osu.edu.